1. Field of the Invention
This invention relates to a toner used in an image-forming process such as electrophotography, electrostatic recording, electrostatic printing or toner jet recording, and to a process for producing toner particles constituting such a toner.
2. Related Background Art
In electrophotography, copied images or printed images are commonly obtained by utilizing a photosensitive member comprised of a photoconductive material, and by forming an electrostatic latent image on the photosensitive member by various means, subsequently developing the latent image by the use of a toner to form a toner image, and transferring via, or not via, an intermediate transfer member the toner image to a transfer material such as paper as occasion calls, and thereafter fixing the toner image to a transfer material by the action of heat, pressure or heat-and-pressure.
As toner productions processes, they are roughly grouped into a pulverization process and a polymerization process. For example, in the process of producing toners by pulverization, at least a binder resin and a colorant are used, and optionally a charge control agent for controlling the triboelectric charge characteristics of toner particles and a release agent are added and mixed. The mixture obtained is melt-kneaded, and the resultant kneaded product is cooled to solidify, which is then made into fine particles by a pulverization means, optionally followed by classification to have a desired particle size distribution to produce toner particles.
As the polymerization process, available are a method in which toner particles are directly produced by suspension polymerization as disclosed in Japanese Patent Application Laid-Open No. 59-61842, and a method of emulsion polymerization in which a monomer composition containing polymerizable monomers, a polymerization initiator, a surface-active agent and further optionally a cross-linking agent, a chain transfer agent and other additives is dispersed in an aqueous medium by means of a suitable stirrer and is simultaneously subjected to polymerization to obtain emulsified resin particles having the desired particle diameter, in the meantime of which a colorant is uniformly dispersed in an aqueous medium containing a surface-active agent, and the resultant dispersion is associated (agglomeration and fusion) with the above emulsified resin particles to obtain toner particles. The toner particles obtained by such polymerization are optionally classified to make adjustments to their desired particle size distribution. The toner particles obtained by such polymerization enable a low-softening substance such as wax as a release agent to be encapsulated in toner particles in a larger quantity than in the pulverization process, and hence have an advantage that the toner particles obtained have superior anti-offset properties.
On the other hand, in the step of polymerization, it is difficult to make the polymerizable monomers react in its entirety and there has been a problem that unreacted polymerizable monomers remain in toner particles. Especially in the case of toner particles produced by suspension polymerization, components having a possibility of inhibiting polymerization reaction as exemplified by a pigment, a charge control agent and/or a magnetic material are present in the polymerizable monomer composition, and hence the unreacted polymerizable monomers tend to remain. This tendency has been remarkable especially when a magnetic material treated with a coupling agent is used.
Where a polymerization initiator is used when the binder resin is produced, a by-product derived from the polymerization initiator may also inevitably be formed. In some cases, the total quantity of such a by-product may unwantedly come larger than the quantity of the unreacted polymerizable monomers.
Organic volatile components such as the unreacted polymerizable monomer and the by-product which are present in the toner particles in a large quantity may lower the fluidity of toner to make work environment bad or may give off an unpleasant smell. Also, where an organic semiconductor is used as a photosensitive member, the use of a toner having toner particles much containing such organic volatile components tends to cause a phenomenon of melt adhesion of toner to the photosensitive member, and problems caused by phenomena of deterioration of the photosensitive member as exemplified by memory ghost and blurred images may arise.
Especially in recent years, as copying machines and printers are made compact and personal, restrictions are more placed on apparatus and a greater load is imposed on the above problems. Also, there is an increasing interest in environment, and it is demanded to reduce any volatile components coming from toner particles, generated in heat-and-pressure fixing assemblies.
Methods by which the total amount of volatile components is made smaller in toner particles may include a method in which they are washed with a highly volatile organic solvent not dissolving binder resins but capable of dissolving the organic volatile components such as unreacted polymerizable monomers and/or reaction by-products; a method in which they are washed with an acid or an alkali; and a method in which a solvent component not dissolving binder resins or a foaming agent is mixed in the binder resin and the toner particles to be obtained are made porous to enlarge the area where the inside volatile components volatilize. However, it is difficult to select solvents because some constituents of the toner particles may dissolve out or any solvent component may remain. Accordingly, in order to make the total amount of volatile components smaller, many studies are made on making treatment to remove them in a drying step after the polymerization for forming the toner particles or binder resin has been completed.
Stated specifically, the following methods are known in the art.
(1) A method in which toner particles are dried by vacuum drying after a dehydration step (Japanese Patent Application Laid-Open No. 8-160662).
(2) A method in which toner particles are vacuum-dried while a gas is injected, after a dehydration step (Japanese Patent Application Laid-Open No. 10-207122).
These methods enable removal of volatile substances, but are undesirable because the rate of reduction of the volatile substances is so low that it may take a long time in order to make the total amount of organic volatile components not more than 500 ppm, preferably not more than 400 ppm, and more preferably not more than 300 ppm, taking account of environment safety. Taking a long time necessitates to use much energy, and hence the production cost for toner particles may greatly increase. Besides, since it takes a long drying time, it consequently follows that thermal and mechanical damage due to stirring is caused to the toner particles in a vacuum dryer. This has tended to affect the surface state of toner particles and tended to produce agglomerates of toner particles.
An object of the present invention is to provide toner particles having solved the above problems, and a process for producing such toner particles.
Another object of the present invention is to provide a process for producing toner particles promising superior developing performance and containing less organic volatile components, and a toner having such toner particles.
Still another object of the present invention is to provide a process for producing toner particles promising superior developing performance and having monomers in a small residue, and a toner having such toner particles.
A further object of the present invention is to provide a process for producing toner particles in a good efficiency which have monomers in a small residue and contain less other organic volatile components, and a toner having such toner particles.
A still further object of the present invention is to provide a process for producing toner particles in a good efficiency which have high fluidity, have good anti-blocking properties and can contribute to formation of good-quality images, and a toner having such toner particles.
The present invention provides a process for producing toner particles which has a polymerization step of polymerizing a polymerizable monomer composition containing at least a polymerizable monomer, in a vessel holding therein an aqueous medium, wherein;
a high-temperature saturated steam with a temperature higher than 100xc2x0 C. is introduced into the aqueous medium held in the vessel, at the latter half of polymerization or after the polymerization has been completed, to remove at least organic volatile components from toner particles having at least a binder resin and a colorant.
The present invention further provides a toner having toner particles containing at least a binder resin and a colorant, wherein;
the binder resin contains as a chief component a vinyl resin selected from the group consisting of a styrene polymer, a polymer of a styrene derivative, a styrene-acrylate copolymer, a styrene-methacrylate copolymer, a styrene-acrylate-methacrylate copolymer and a mixture of any of these; the total amount of organic volatile components contained in the toner, in terms of toluene based on the weight of the toner is 500 ppm or less and the residue of vinyl monomers contained in the toner, in terms of toluene based on the weight of the toner is 75 ppm or less, according to the analysis of the organic volatile components by the head space method at a heating temperature of 150xc2x0 C.; and the toner has an average circularity of 0.950 or more.